The development of the first cold cathode pressure gauge based on the Penning cell (U.S. Pat. No. 2,197,079) which could only operate up to pressures of around 10−6 mbar, and the resulting commercial use of pressure gauges, prompted several patents, which are briefly discussed hereinbelow.
A pressure gauge suitable for measuring pressures in the ultrahigh-vacuum range, which could measure pressures of up to 10−12 mbar, was obtained by providing the ions collector with a special protection, thereby reducing the influence of parasitic currents, which were a consequence of the intense electric field inside the cell (U.S. Pat. No. 3,051,868).
The problem of weak currents at very low pressures was overcome by employing an additional source of ions (U.S. Pat. No. 5,278,510). As a source of ions a micropoint cathode is used which is arranged outside of an anode grid, so that electrons can pass into the interior of the anode cylinder and assist the ionization of molecules. The ion current flow is thereby increased and, consequently, easier to measure.
By developing a pressure gauge operating with a constant voltage at low pressures, and with a periodically varying voltage at higher pressures, a wider operating range of pressure measurement was achieved (U.S. Pat. No. 4,000,457). A pressure gauge of this type can operate even at high pressures of up to 10−1 mbar.
Furthermore, a pressure gauge was devised having a magnetic field that varies in direction along the gauge cell's axis, thereby facilitating the ignition of the pressure gauge at low pressures, which likewise broadens the operating pressure range of the pressure gauge (U.S. Pat. No. 5,568,053).
To shorten the ignition time of the pressure gauge at low pressures, an additional UV light source was introduced, triggering an emission of photoelectrons which, in turn, help initiate the electrical discharge (U.S. Pat. No. 5,198,772).
In U.S. Pat. No. 4,866,640 a pressure gauge is disclosed which, in addition to measuring the ion current, also measures the concomitant gas temperature. Said pressure gauge includes a special microprocessor which temperature-compensates the pressure readout in view of separately stored calibration data.
Prior art cold cathode pressure gauges operate at a constant voltage, which is situated outside of the maximum. For this reason, the ion current is non-linear. Moreover, instabilities and discontinuities often occur, impeding measurement.
It is an object of this invention to provide a method and a device for measuring ultrahigh vacuum and, more particularly, to provide a method for measuring ultrahigh vacuum by means of an ultrahigh-vacuum cold cathode pressure gauge and, as a further object of the invention, to provide an ultrahigh-vacuum cold cathode pressure gauge, that avoids prior art disadvantages, exhibits a better sensitivity, and enables linear, stable and continuous measurement of pressure in the ultrahigh-vacuum range.
According to the present invention the object is achieved by a method and a device for measuring ultrahigh vacuum according to the independent patent claims.